The present disclosure relates, in general, to optical range finders and, in particular, to time of flight optical range finders.
Range finding refers to a technique for determining the range, or distance, to a target and can be carried out using light pulses having a short duration. For example, a range finder can direct a burst of light, such as laser light, toward a target. The light strikes the target and propagates back toward the range finder. The time required for the light to travel from the range finder to the target, and then back to the range finder is measured by the range finder and is used to determine the target""s range, in other words, the line-of-sight distance from the range finder to the target. The range from the source to the target is proportional to the speed of the light.
Range finding can be used in various industrial applications, such as machine vision. For example, a machine vision system with range finding capability can be used for machine safety. Such systems generally include a lighting system to illuminate the target and a camera for capturing light reflected from the target. The camera can include an array of photo-sensitive pixels.
For ranges on the order of several feet or less, the total travel time of the light is on the order of about several nanoseconds (nsec). It is often desirable to determine the range of the target with an accuracy of within several inches. Such accuracy requires that the travel time of the light be measured to within about one nanosecond or less. Unfortunately, photo-sensitive pixels, such as CMOS active pixel sensors (APS), may have inherent speed limitations on the order of about 5-10 nsec. Therefore, techniques are needed for measuring the duration between the beginning of the light pulse and the arrival of light reflected from the target where the duration may be less than the inherent speed limitation of the pixel.
In general, according to one aspect, a method of determining a range of an object includes emitting light toward a target and sensing light reflected by the target. Signals corresponding to the sensed light are integrated during multiple integration periods. Each integration period is different from other integration periods. A range of the target can be calculated based on the integrated signals.
In various implementations, the integration periods may differ in duration, in the respective time frames over which they extend, or both. The integration periods may be shifted or offset from one another in time and may overlap one another.
Range finders for performing the foregoing techniques also are disclosed. According to one aspect, a range finder includes a light source arranged to emit light toward a target and a photo-sensitive pixel that receives light reflected by the target. The range finder includes a readout circuit which can be coupled to an output of the pixel. The readout circuit includes multiple integrator circuits in parallel. A controller can receive output signals from the readout circuit and is configured to control the readout circuit so that each integrator circuit in the readout circuit provides an integrated signal based on a pixel output signal using an integration period that is different from integration periods of other integrator circuits in the same readout circuit. The controller is further configured to calculate a range of the target based on the integrated signals obtained from the integrator circuits.
Some implementations include one or more of the following features. The controller can be configured to control the readout circuit so that the integrator circuits begin to integrate the pixel output signal at substantially the same time as one another and wherein outputs of the integrator circuits are sampled at different times from one another. Alternatively, the controller can be configured to control the readout circuit so that the integrator circuits begin to integrate the pixel output signal at different times from one another. Outputs of the integrator circuits also can be sampled at different times from one another. The integration times corresponding to the sampled outputs of the integrator circuits in the particular readout circuit can partially overlap one another.
The controller can be configured to calculate a centroid value based on the integrated signals obtained from the integrator circuits and to calculate the range of the target based on the centroid value.
Each integrator circuit can include a charge transimpedance amplifier circuit having an output coupled to a respective sample and hold circuit. The pixel output signal can be coupled to the charge transimpedance amplifier circuit via an operational amplifier follower. The charge transimpedance amplifier circuit can include an operational amplifier having an integration reset switch and a first capacitor coupled in parallel between an output of the operational amplifier and a first input of the operational amplifier. Additionally, the pixel output signal can be coupled to the charge transimpedance amplifier circuit via an operational amplifier follower. An output of the operational amplifier follower can be coupled to the first input of the operational amplifier through a second capacitor.
According to another aspect, a range finder includes a light source arrange to emit light toward a target and multiple photo-sensitive pixels that receive light reflected by the target. A readout circuit can be selectively coupled to a respective output of any one of the pixels. The range finder includes a controller coupled to an output of the readout circuit. The controller is configured to provide control signals to the pixels to establish a respective integration period for each pixel during which the pixel integrates light reflected by the target. The integration period for each pixel is different from the integration periods of other pixels. The controller is configured to provide control signals so that an output signal of each pixel is sent to the controller via the readout circuit following the integration period of the respective pixel. The controller is further configured to calculate a range of the target based on the pixel output signals received via the readout circuit.
Some implementations include one or more of the following features. The controller can provide control signals so that the integration times of the pixels start at substantially the same time as one another and end at different times from one another. Alternatively, the controller can provide control signals so that the integration times of the pixels start at different times from one another. The controller also can provide control signals so that the integration times of the pixels end at different times from one another. The integration times of the pixels can partially overlap one another.
Additionally, the controller can be configured to calculate a centroid value based on the pixel output signals received via the readout circuit and to calculate the range of the target based on the centroid value.
Each pixel can include, for example, a photogate-type or photodiode-type sensor. In some implementations, the range finder includes a lens arranged to direct the light reflected by the target substantially equally among the pixels.
Various implementations include one or more of the following advantages. The foregoing techniques, described in greater detail below, can be used to measure the duration between the beginning of the light pulse and the arrival of light reflected from the target even where the duration may be less than the inherent speed limitations of the pixel(s). Thus, range finders with greater accuracy can be provided. In particular, the range of a target can be determined with high accuracy even if the range is relatively small, in other words, if the target is relatively close to the range finder such that the time of flight of the pulse is on the order of about a nanosecond or less.
Other features and advantages will be readily apparent from the following description, accompanying drawings and the claims.